This invention relates to high intensity discharge lamps and more particularly to high intensity discharge metal halide lamps. Still more particularly it relates to a metal halide filling for ceramic metal halide lamps. Ceramic metal halide lamps usually contain TlI and NaI in their filling. However, other known metal halide materials such as DyI3, HoI3, and TmI3 are frequently used.
This invention relates generally to high intensity discharge (HID) lamps and, more particularly, to metal halide lamps with ceramic discharge vessels having superior dimming characteristics. Low wattage metal halide lamps with their high efficacy have become widely used for interior lighting. Until now, almost all metal halide lamps were used for general lighting and have been operated at rated power. Due to the ever-increasing interest in energy conserving lighting systems, some dimmable metal halide ballast systems are available on the market for metal halide lamps. Working under dimmed conditions (usually dimmed to as low as 50% of rated power), the performance of the regular metal halide lamps on the market deteriorate dramatically. Typically the color temperature (CCT) increases significantly, while the color-rendering index (CRI) decreases. And the lamp hue will deteriorate from white to greenish or pinkish depending on the lamp""s chemistry. Furthermore the efficacy of the lamp usually decreases significantly.
Under dimming conditions, the light emitted by commercially available metal halide lamps will have very strong green hue, which can be very objectionable for many indoor applications. The strong green hue in the light of dimmed ceramic metal halide lamp is due to the radiation of Tl green lines (535.0 nm). Under dimming conditions, the discharge tube wall temperatures as well as its cold-spot temperature is much lower compared to the temperatures at rated power. At the lower cold-spot temperatures under dimming conditions, the ratio of partial pressure of TlI in the discharge tube is much higher compared to the partial pressures of other metal halides. Under dimming conditions, the relatively higher TlI partial pressure emits relatively stronger green Tl radiation at 535.0 nm. Since the Tl radiation at 535.0 nm is very close to the peak of the human eye sensitivity curve, higher lumen efficacy is achieved at rated power with TlI as one of the filling components in almost all commercial ceramic metal halide lamps.
With the present invention, superior lamp performance under dimming conditions with ceramic discharge vessel was achieved in nitrogen filled outer jackets at relatively high pressure between about 350 and 600 mmHg by a new chemical fill of the ceramic discharge tubes. In the newly invented lamps, MgI2 is used in the discharge tubes to replace the TlI in the fill composition of ceramic metal halide lamps. MgI2 is used to replace the TlI as one of the fill components because Mg has both green radiation for higher efficacy and has a similar vapor pressure variation with temperature as that of the rare earth iodides in the discharge tube dosing.
Due to the similar vapor pressure variation with temperatures, MgI2 partial pressure will drop under dimming conditions proportionally to that of the other rare-earth halides. This leads to a white lamp under dimming rather than the greenish hue of the lamps with TlI.
Also, the relatively higher vapor pressure of MgI2 at rated power results in relatively strong green radiation at 518 nm. Since the Mg radiation at 518.0 nm is very close to the peak of the human eye sensitivity curve, higher lumen efficacy is achieved at rated power with MgI2 as one of the filling components. (Under some circumstances MgBr2 could be substituted for TlI).
Therefore an objective of the present invention is to provide a metal halide lamp that when dimmed to about 50% power retains substantially its white hue.
Another objective of the present invention is to provide a metal halide lamp that when dimmed to about 50% power retains the CCT (correlated color temperature) substantially as in rated power.
Yet another objective of the present invention is to provide a metal halide discharge tube fill formulation that at rated power gives substantially similar performance (including efficacy, CRI, CCT and Duv) as the currently available products on the market.
Another objective of the present invention is to provide a metal halide lamp whose performance does not deteriorate under dimming, and whose outer jacket is filled with a gas at high pressure so that arcing is avoided at the end of life or if the outer jacket leaks during the lamp life.
Still another objective of the present invention is to provide a metal halide lamp that when dimmed to about 50% power its color-rendering index remains above 70.
Disadvantages of existing metal halide discharge lamps:
1. Existing metal halide lamps are optimized for a rated wattage without consideration of dimming performance.
2. When lamp power is reduced to about 50% of rated value the correlated color temperature increases dramatically often more than 1000xc2x0 K. This change is not acceptable for most indoor applications.
3. When lamp power is reduced to about 50% of rated value the color rendering index decreases significantly.
4. When lamp power is reduced to about 50% of rated wattage the light radiated by the regular metal halide lamp has a color point, which is far away from the black body line, leading to a nonwhite hue.
There is no known publication on the filling materials of metal halide lamps with the purpose of improving dimming performance of metal halide lamps.
U.S. patent application Ser. No. 09/074,623 filed May 7, 1998 now U.S. Pat. No. 6,242,851 by Zhu et. al. by the same assignee, was filed on an invention of a new metal halide lamp which has significantly better lamp performance under dimming conditions. In that patent application, a lamp has a discharge tube burning in vacuum outer jacket to reduce convection heat loss from the cold-spot of the discharge tube, and a metal heat shield is used on the discharge tube to reduce radiation heat loss from the cold-spot during dimming. The invention shows very good dimming performance due to the fact that the thermal emissivity of the metal shield is much lower than that of a ceramic surface. Also the emissivity of the metal goes down as the temperature drops thereby keeping the cold-spot and vapor pressure of the salts substantially constant. A disadvantage of the invention is that widely used high voltage starting pulses on low wattage metal halide lamps in conjunction with a vacuum jacket may make the lamp susceptible to arcing when discharge tube leaks or slow outer jacket leaks exist.
U.S. Pat. No. 5,698,948 discloses a lamp that contains halides of Mg, Tl and one or several of the elements from the group formed by Sc, Y and Ln. The lamp filling also contains Mg to improve lumen maintenance. The lamp has a disadvantage of strong green hue when dimmed to lower than the rated power, due to the relatively higher vapor pressure of TlI under dimming conditions.
Lamps according to the present invention do not contain TlI in their chemical fill, so there is no hue change due to higher TlI vapor pressure under dimming conditions.
Lamps according to the present invention contain MgI2 as one of the main filling materials. The MgI2 is in a molar quantity between about 5 and 50% of the total molar quantity of the total halides. It replaces TlI for green light emission and to reach the same lumen efficacy as the commercial lamps containing Tl fills. The lamp, according U.S. Pat. No. 5,698,948, contains MgI2 as an addition to the filling ingredients just to improve lumen maintenance during lamp life. Through the addition of Mg to the lamp fill, according to the patent, one can influence the balance of one or several chemical reaction between Sc, Y and Ln with spinel (MgAl2O4) to such an extent that this balance is already achieved shortly after the beginning of lamp life, after which a further removal of the ingredients Sc, Y and Ln does not take place. Since the Mg addition is for reducing chemical reaction between the filling ingredients and the wall, the quantity of Mg fill is based on the surface area of the inner wall of the discharge vessel.
Since MgI2 fill in the present invention is for light emission and for better lamp performance under dimming conditions, the optimization of the quantities of MgI2 fill are based on the lamp performance under rated power as well as reduced power conditions, rather than the surface area of the discharge vessel.